1. Field of the Invention
The present invention relates in general to ballasts for fluorescent lamps, and more particularly to a digital controlled electronic ballast with a piezoelectric transformer which is capable of supplying high power of the 28-Watt class to the ballast, whereby the ballast automatically controls precisely the brightness of a lamp in a digital frequency modulation manner.
2. Description of the Prior Art
Ballasts for fluorescent lamps are generally classified into a self-excited type for determining a lamp drive oscillation frequency depending on a value determined by passive elements such as a coil, capacitor and the like, and a separate-excited type for performing frequency modulation in a digital manner to improve the precision of frequency modulation based on an analog manner. Ballasts of the separate-excited type are adapted to control the brightness of lamps in a frequency modulation manner. In this regard, ballasts of the separate-excited type have recently been used more than those of the self-excited type in that the ballasts of the self-excited type have many limitations as well as such a large amount of high-frequency components as to jam the surrounding electronic equipment.
However, existing ballasts have a disadvantage in that they are low in precision because they are controlled in an analog manner.
A conventional ballast of the separate-excited type basically comprises a rectifier for rectifying a commercial alternating current (AC) voltage to convert it into a direct current (DC) voltage, a power factor improvement circuit for correcting a power factor of an output DC voltage from the rectifier such that the output DC voltage is always constant in level irrespective of a variation in an input voltage to the rectifier, an inverter for performing a switching operation in response to an input frequency signal to convert a power factor-improved DC voltage from the power factor improvement circuit into an AC voltage of a certain frequency, and a series resonance circuit for performing a resonance operation in response to the AC voltage converted by the inverter to drive a lamp. The resonance circuit is typically composed of a coiled transformer. A capacitor is connected to the coiled transformer in such a way that a resonance occurs owing to an inductance of the transformer and a capacitance of the capacitor.
However, the coiled transformer is subject to a severe material deviation in core during its fabrication process, resulting in the occurrence of a high defective rate amounting to several tens percentages in actual practice. This high defective rate in turn results in a degradation in reliability of the ballast. Further, the use of the coiled transformer necessitates the arrangement of a lamp capacitor at a lamp stage, thereby increasing the number of components and in turn the cost.
Because of such problems, a piezoelectric transformer has gradually replaced the coiled transformer, and currently applied to even a cold cathode lamp inverter and low-power fluorescent lamp ballast.
However, existing piezoelectric transformers provide low power of the maximum 18-Watt class. For this reason, a fluorescent lamp ballast employing such a piezoelectric transformer must essentially comprise a charge pump capacitor and lamp capacitor for power factor compensation. As a result, existing piezoelectric transformers have not been applied yet to high-power fluorescent lamps.
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide a digital controlled electronic ballast with a piezoelectric transformer which is capable of supplying high power of the 28-Watt class to the ballast, whereby the ballast automatically controls precisely the brightness of a lamp in a digital frequency modulation manner and applies stable voltage and current to the lamp.
In accordance with the present invention, the above and other objects can be accomplished by the provision of a digital controlled electronic ballast for turning on/off a lamp, comprising a rectifier for rectifying an input low-frequency alternating current (AC) voltage to convert it into a direct current (DC) voltage; a power factor improvement circuit for compensating a power factor for a variation in the input low-frequency AC voltage to maintain the level of an output DC voltage from the rectifier constant; an inverter for performing a switching operation for a power factor-improved DC voltage from the power factor improvement circuit to convert it into a high-frequency AC voltage; a resonance circuit connected to the output of the inverter for transferring the high-frequency AC voltage from the inverter to the lamp, the resonance circuit including a coil and a high-power piezoelectric transformer; and a digital controller for modulating a frequency controlling a switching period of the inverter when the lamp is turned on/off, to control an intensity of illumination of the lamp.
Preferably, the high-power piezoelectric transformer may be made of a composition expressed by the following chemical formula:
Pb1-aSra[(Nixc2xdWxc2xd)b(Mn⅓Nb⅔)c(Zr1-xTix)l-b-c]O3+kPbO
where, a is 0xcx9c0.06 mol, b is 0.01xcx9c0.05 mol, c is 0.01xcx9c0.09 mol, x is 0.47xcx9c0.53 mol and k is 0.1xcx9c10.7 wt.
Further, preferably, the high-power piezoelectric transformer may include a substantially hexahedral piezoelectric body block; a substantially rhombic input electrode formed on a central portion of each of the upper and lower surfaces of the piezoelectric body block; and an output electrode formed on each of the upper and lower surfaces of the piezoelectric body block such that it surrounds the input electrode while being spaced from the input electrode at a regular interval.